Radiation sensing arrangements

Apparatus for checking for obscuration of a radiation-transmitting window capable of transmitting radiation from relatively hot sources but not from relatively cold sources and which is used in a radiation detection system incorporating radiation sensing means capable of responding to radiation from relatively hot and from relatively cold sources and arranged to sense radiation passing through the said window, the apparatus comprising

• a relatively cold source producing testing radiation,
• means for directing the testing radiation to the radiation sensing means from the opposite side of the said window to the radiation sensing means,
• the path of the testing radiation by-passing the said window but passing adjacent thereto, and
• means for monitoring the output signal produced by the radiation sensing means in response to the received testing radiation so as to assess the degree of the said obscuration.

A fire or explosion detection arrangement, comprising

• a housing having first and second adjacent radiation-transmitting windows,
• the first radiation transmitting window including a radiation transmitting filter having a passband corresponding to a predetermined wavelength band,
• a radiation sensor mounted within the housing so as to receive radiation from a fire or explosion external to the housing through the first window,
• the predetermined passband corresponding to a wavelength band within which a fire or explosion to be detected generates radiation,
• electrical circuitry connected to the radiation sensor and responsive to the said radiation received thereby to produce an output signal accordingly,
• a source of testing radiation mounted externally of the housing and energisable to generate testing radiation having a wavelength or wavelengths capable of passing through the second transmitting window but not through the first window,
• means for directing the testing radiation through the second window to the radiation sensor,
• means responsive to the level of the testing radiation received at the sensor for producing a corresponding electrical signal which is fed through the said electrical processing circuitry, and
• means responsive to the electrical signal so fed through the electrical processing circuitry to determine whether the level of obscuration of the second window lies above or below a predetermined level, so as to make an assessment whether the level of obscuration of the first window lies above or below a predetermined level.